1. Field of the Invention
This invention relates to an optical film and a process for producing the same, and more particularly to an optical film having a high refractive index, suitable as an anti-reflecting film for opto-magnetic disks and optical disks, and to a process for producing such optical films.
2. Description of the Prior Art
As regards the prior art for producing optical films, there are known, for example, the process disclosed in U.S. Pat. No. 3,934,961, wherein an Al.sub.2 O.sub.3 -ZrO.sub.2 mixed sinter is used as a raw material for vapor deposition in vacuo on a glass or plastic substrate, and the process disclosed in Japanese Laid-open Patent Publication No. 35,211/1975, wherein a ZrO.sub.2 -TiO.sub.2 mixed sinter or TiO.sub.2 alone is used as a raw material for vapor deposition in vacuo on a glass or plastic substrate.
However, the optical film formed from an Al.sub.2 O.sub.3 -ZrO.sub.2 or ZrO.sub.2 -TiO.sub.2 sinter by vapor deposition on a substrate kept at a temperature of about 20.degree.-80.degree. C. has a refractive index not sufficiently high but equivalent to that of ZrO.sub.2 film.
On the other hand, an optical film of high refractive index can be formed by a process similar to the above but by keeping the substrate at a temperature of 300.degree. C. or higher. However, it is undesirable that, in the process for producing opto-magnetic disks or optical disks, recording layers or heat insulation layers of methacrylic resin, rigid vinyl chloride resin, polycarbonate resin, urea resin, or polyethylene resin, supported on substrates be heated to a temperature of 300.degree. C. or higher. In particular, during the formation of an optical film on such a heat insulation layer, the surface of this layer heated at or above 300.degree. C. is liable to undergo oxidation, this being responsible for the development of "fog". Also, the recording layer on heating loses sometimes the ability to respond to laser beams.
Among optical films formed by vapor deposition on substrates kept at room temperatures, ZrO.sub.2. Ta.sub.2 O.sub.5 and Nb.sub.2 O.sub.3 films exhibit refractive indexes as high as about 1.9, 2.0 and 2.1, respectively. These optical films can be formed on substrates by bombarding raw materials, ZrO.sub.2, Ta.sub.2 O.sub.5 and Nb.sub.2 O.sub.5 sinters, for vapor deposition with electrons from an electron gun in vacuo. However, these raw materials, during the bombardment, scatter and then adhere in the form of fine particles onto the deposition film. This is a cause of the high percentage of defective products in the production of these films.